The present invention relates to master cylinders, of the type with valves, and which are intended in particular to equip hydraulic braking circuits of motor vehicles.
A valve-type master cylinder consists, in the conventional way, of a bore formed in a body and in which there slides at least one primary piston provided with a compensation passage capable of placing a pressure chamber of the master cylinder in communication with a reservoir of fluid at low pressure when the master cylinder is in its position of rest, its communication being interrupted by a second piston upon actuation of the master cylinder by a control rod acting on the second piston sliding in the bore of the master cylinder, a first compression spring urging the primary piston towards a rear position of rest, a second compression spring being arranged between the primary piston and the second piston and urging the latter towards a rear position of rest, the preload at rest of the first compression spring exceeding the preload at rest of the second compression spring.
A valve-type master cylinder of this kind is known, for example, from document GB-B-1,155,160. In this known master cylinder, the primary piston rests, in the position of rest, on a stop formed at the end of a screw interacting with a tapped bore formed in the body of the master cylinder, while the second piston rests, in the position of rest, on an annular stop kept at the rear end of the bore of the body of the master cylinder.
Upon actuation of such a master cylinder via the control rod, the second piston is lifted off its rear stop to move to seal the compensation passage so as to interrupt the communication between the pressure chamber of the master cylinder and the reservoir of fluid at low pressure, and so that the pressure in the pressure chamber can increase.
This travel of the control rod needed to achieve closure of the compensation passage is known in the field of braking by the name of master cylinder dead travel. Depending on the manufacturing tolerances on the various elements of which a master cylinder is composed, it will be understood then that the dead travel can vary across a production run. Furthermore, with the objective of obtaining a braking action as soon as possible after the beginning of the actuation of the master cylinder, it is very important on the one hand for its dead travel to be as small as possible and on the other hand for it to be possible for this travel to be set to this minimum value when the master cylinder is assembled and for it to keep this minimum value during use of the master cylinder.
In the master cylinder known from the aforementioned document, the dead travel is determined by the positions of rest of the primary piston and of the second piston are determined respectively by a stop screw and an annular stop, the relative positions of which can vary over the course of time or over the course of the operation of the master cylinder. [sic].
The object of the present invention is therefore to provide a master cylinder in which the dead travel can be set simply to a minimum value and which keeps this minimum value throughout the use of the master cylinder, it being necessary also for this master cylinder to be obtained using means which are simple and quick to employ in a reliable and inexpensive way.
According to the present invention, the rear positions of rest of the primary piston and of the input piston are defined by the same stop integral with the body of the master cylinder.
Other objectives, features and advantages of the present invention will emerge more clearly from the description which follows of one embodiment given by way of illustration with reference to the appended drawings.